advances in fuel cells - chapter 2

Research Signpost 37/661 (2), Fort P.O., Trivandrum-695 023, Kerala, India

Advances in Fuel Cells, 2005: ISBN: 81-308-0026-8 Editor: Xiang-Wu Zhang

Privileged materials for direct methanol fuel cells

Loka Subramanyam Sarma and Bing Joe Hwang Nanoelectrochemistry Laboratory, Department of Chemical Engineering National Taiwan University of Science and Technology, Taipei 106, Taiwan Republic of China

Abstract Direct methanol fuel cells (DMFCs) have a

continuous supply of methanol as the fuel to the anode and convert chemical energy directly into electrical energy with high-energy density and low emission of pollutants. DMFCs are about to come of age as power sources for electric vehicle, transport applications and portable electronic devices including lap tops, cell phone power packs, generators, and other power-hungry products. One of the major challenges in the commercialization of DMFCs is the high cost of noble metal-based electrocatalysts and state-of-the-art

Correspondence/Reprint request: Dr. Bing Joe Hwang, Nanoelectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan, Republic of China. E-mail: [email protected]

Loka Subramanyam Sarma & Bing Joe Hwang 2

Nafion membranes. In this regard major research efforts have been directed towards finding innovative materials with attributes such as decreased catalyst loadings, optimum fuel and proton access to the active sites, electronic continuity in the carbon supports and low methanol permeability in the proton exchange membranes. In this chapter we described the recent progress including our own work in search and development of innovative electrocatalysts both for methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR), carbon supports, and proton exchange membranes alternative to Nafion. 1. Introduction

Direct methanol fuel cells (DMFCs) are promising energy sources for vehicle and portable devices because of its high-energy conversion efficiency, low-to-zero pollutant emission, methanol fuel availability, high-energy density of the fuel (6000 Wh/kg), and low operating temperatures (60 to 100 oC) [13]. When compared to the emissions from conventional internal-combustion-engined vehicles (ICEVs), the emissions from DMFCs at an operating temperature range from 60 to 80 oC are significantly lower. Apart from the principle emitters such as CO2 and H2O traces of formaldehyde can also be found. However, due to the inherent higher efficiency of DMFCs, the extent of carbon monoxide emission is substantially lower than that from ICEVs [4]. With the strict environmental laws and due to the increased concerns on the consequences of fossil fuel usage in power generation and propulsion of vehicles, much efforts have been focused on the development of powerful, clean and finely distributed power generation and development of DMFCs comes under this aim. Despite the significant progress, DMFCs still suffer from many obstacles such as low power density, which has been attributed to the poor kinetics of both anode [5], and cathode [6], high flux of water/methanol across membranes [7], and mixed potential at cathode [8]. These phenomena lead to high overpotentials involving methanol cross-over overpotential, activation overpotential, and concentration overpotential at both the anode and the cathode sides and, hence, reduction in cell voltage. Other important constraints are the high cost of noble metal catalysts and perfluorosulfonic membranes and higher production costs of the various components of the device.

A great deal of effort has been devoted over the years to the development of innovative materials for DMFCs and in this chapter; we describe recent progress in the development of carbon supports, electrocatalysts both for methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR), and proton exchange membranes alternative to state-of-the art Nafion membrane for DMFCs. Before discussing these issues in details we will discuss briefly

Privileged materials for direct methanol fuel cells 3

the two principle reactions in DMFCs such as methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) in the following paragraphs.

2. Principle reactions in DMFCS The principle and schematic diagram of a DMFC is shown in Figure 1. In

a typical DMFC, methanol and water molecules are simultaneously electro-oxidized at anode to produce CO2, electrons and protons through the reaction (methanol oxidation reaction, MOR): CH3OH + H2O CO2 + 6H+ + 6e [Ea = 0.02 V vs. SHE] (1)

Protons generated at the anode pass through the proton exchange membrane to cathode and combine with the electrons and the oxidant air or oxygen simultaneously reducing to water as (oxygen reduction reaction, ORR): 3/2O2 + 6H+ + 6e 3H2O [Ec = 1.23 V vs. SHE] (2) Reactions (1) and (2) can be combined to give the overall reaction: CH3OH + 3/2O2 CO2 + 2H2O [Ecell = 1.21 V] (3)

The free energy (G) of overall reaction at 25 oC and 1 atm is 686 kJmol1CH3OH [9].

The free energy (G) of overall reaction at 25 oC and 1 atm is 686 kJmol1CH3OH [9].

Figure 1. Schematic drawing and principle of a DMFC single cell depicting H+, H2O and CH3OH transport through the proton exchange membrane (PEM).


2.1. Methanol oxidation reaction (MOR) The oxidation of methanol and water molecules is the principle reaction

occurring at anode of DMFCs. Pt is the best material for the adsorption and dehydrogenation of methanol. However, formation of intermediate species such as CO [10], formic acid and formaldehyde poison the platinum anode and impede the catalytic performance for methanol oxidation. Among them poisoning from CO species is severe on Pt electrode surface during the electro-oxidation of methanol. The detailed methanol oxidation mechanism for the oxidation of methanol at Pt electrodes, involving the adsorption of CH3OH and its successive dehydrogenation, yielding linearly bonded CO, has been proposed by Beden et al. [11]. The methanol is adsorbed on Pt surface (equation 4) and then undergoes a sequence of dehydrogenation steps to yield linearly bonded CO species (equations 5a to 5d), as represented below:

Pt + CH3OH Pt (CH3OH)ads (4) Pt (CH3OH)ads Pt (CH3O)ads + H+ + e (5a) Pt (CH3O)ads Pt (CH2O)ads + H+ + e (5b)

Pt (CH2O)ads Pt (CHO)ads + H+ + e (5c) Pt (CHO)ads Pt (CO)ads + H+ + e (5d)

On the pure Pt surface water dissociation will occur only at higher anodic overpotentials to form Pt(OH)ads species (equation 6): Pt + H2O Pt(OH)ads + H+ + e (6) The last step is the reaction of PtOH species with Pt-adsorbed CO to give carbon dioxide (equation 7): Pt (CO)ads + Pt(OH)ads 2Pt + CO2 + H+ + e (7)

At potentials of technical interest for DMFCs (


M(H2O)ads M(OH)ads + H+ + e (9) Pt (CO)ads + M(OH)ads Pt + M + CO2 + H+ + e (10)

During methanol oxidation the efficient catalyst must allow a complete

oxidation to CO2. Currently, carbon-supported PtRu catalysts have been shown to be among the best candidates for electrochemical oxidation of methanol at anodes of DMFCs [15, 16]. A Pt/Ru atomic ratio of 1:1 was reported as the preferred atomic composition [17, 18].

The enhanced electrocatalytic activities enabled by promoter metals such as Ru, Sn, Mo or Os in mixed PtM catalysts have been explained by the so-called bifunctional mechanism [13, 14, 19, 20] and the ligand (electronic) mechanism [2124].

In the bifunctional mechanism the promoter metal is oxophilic and thought to provide sites for water adsorption thereby increasing the methanol oxidation rate through bimolecular surface reaction. In the ligand effects the promoter metal can alter the electronic state of the Pt by contributing d-electron density, facilitates weakening of PtCO bond and thus easy removal of CO is expected. Lu and Masel have noticed that bifunctional effect has a larger effect on CO removal than the ligand effect when they used 0.25 monolayers of Ru deposited on Pt (110) surface in ultra-high-vacuum (UHV) conditions. They found that out of the 46 kcal/mol (170260 meV) reduction in the potential for CO removal, only about 1 kcal/mol (40 meV) is associated with the ligand effect, whereas 35 kcal/mol (130 to 220 meV) is associated with the bifunctional mechanism [25].

With the advent of computational methods more fundamental insights into the CO oxidation mechanism can be obtained. Ishikawa et al have studied the adsorption of CO on PtM metal surfaces (M = Ru, Sn or Ge) with the relativistic density-functional self-consistent field X method. From their results it was found that the presence of M atoms weakens the PtC bond, and generally slightly lowers the CO stretching frequency of adsorbed CO. Substitution of Ru and Sn for platinum is found to alter the dissociation energy of H2O. The results indicate that the promoting effect of alloying atoms involves both modification of PtCO binding and water activation [26]. 2.2. Oxygen reduction reaction (ORR)

The electroreduction of O2 is a multielectron reaction and depending on the experimental conditions it is known to take place through two different reaction pathways: the direct four-electron pathway (equation 11) in which O2 is reduced directly to water and the two-electron pathway (equation 12) in which O2 is reduced to peroxide followed by the decomposition or further reduction to water [2729]


O2 + 4 H+ + 4 e 2 H2O (E0 = 1.23 V vs. SHE) (11) O2 + 2 H+ + 2 e H2O2 (E0 = 0.67 V vs. SHE) (12)

Platinum has long been the best electrocatalyst for ORR, and oxygen reduction over platinum and other Pt bimetallic surfaces has been reviewed recently [3032]. As discussed in these reviews, over the three low-index (100), (111), and (110) platinum surfaces in aqueous perchloric and sulphuric acid, the complete four-electron reduction to water can be observed. Two electron reduction to hydrogen peroxide occurs on the Pt (111) surface in the hydrogen adsorption region which is an evident of surface site blocking by H(ads). Equally OH(ads), which begins forming from water decomposition at about 0.6 V on platinum electrodes, is also believed to inhibit oxygen reduction by blocking surface sites, and it may contribute to the need to operate oxygen electrodes at several millivolts relative to the 1.23 V reversible potential on the standard hydrogen scale.

On Pt electrode surface the adsorption and reduction of O2 via four-electron reduction pathway follows the steps (equation 13a13d): PtO2 + H+ + e PtOOH (13a) PtOOH + H+ + e PtO + H2O (13b)

PtO + H+ + e PtOH (13c) PtOH + H+ + e PtOH2 (13d)

Sidik and Anderson have applied density functional theory to study the four-electron reduction mechanism of O2 on platinum in aqueous acidic electrolytes [33]. Their studies suggest that the first electron transfer step (equation 13a) is the rate determining step. They have calculated the activation energy for the rate determining step which is of about 0.60 eV at 1.23 V and close to the experimental value of 0.44 eV on Pt (111) in H2SO4. In the two-electron reduction pathway the O2 reduction follows the steps (equation 14a14b): PtO2 + H+ + e PtOOH (14a)

PtOOH + H+ + e Pt(OHOH)ads (14b)

The adsorbed peroxide in equation 14b can be electrochemically reduced to water (14c14d):


Pt(OHOH)ads + H+ + e Pt(OH) + H2O (14c)

Pt(OH) + H+ + e PtOH2 (14d) or catalytically decomposed on the electrode surface, or can be desorbed into the bulk of the solution. Although a number of important problems pertaining to the interpretation of the reaction pathway for the ORR on Pt (hkl) have not yet resolved, recent studies of Markovic et al suggest that a series pathway via an (H2O2)ad intermediate may be operative on Pt and Pt bi-metallic surfaces [30, 3436]. Because of the high cost of platinum and the slow kinetics of the oxygen reduction, many efforts have been directed on finding alternative catalysts [37, 38]. Studies involving partial substitution of Pt with other transition metals like Co, Cr and Ni have indicated that these alloyed materials showed better performance towards ORR [3941]. Even though significant progress has been achieved in improving the ORR kinetics, mixed potential at the cathode due to the methanol cross over from anode makes the overpotential of this reaction at the desired current densities (e.g. 500 mA cm2) about 420450 mV in DMFCs. 3. Privileged materials for DMFCS

At present the two most technical challenges for commercialization of DMFCs are the high cost of noble metals used in both anode and cathode materials and methanol crossover to the cathode [42, 43]. One way to solve the issue of high cost of noble metals is to decrease the amount of Pt used in DMFC catalysts via the increase of the utilization of Pt and this attains priority research during the past decade. For the effective utilization of the Pt-based catalyst, the Pt must have simultaneous access to the fuel, the electron and the proton-conducting mediums. In general this could be achieved by blending the carbon-supported Pt catalyst and proton conducting Nafion solution in a skillful manner. However, Pt utilization rates are at only 2030% even in commercially available advanced electrodes. Efforts directed at improving the utilization efficiency of the Pt catalyst have focused on finding the optimum material configurations while minimizing the Pt loading and satisfying the requirements of gas access, proton access, and electronic continuity. In order to avoid the mixed potential at cathode, methanol-tolerant cathode catalysts need to be developed.

3.1. Carbon supports During the recent years many efforts are aimed to decrease the amount of

Pt used in a DMFC via the increase of the utilization efficiency of Pt by


finding innovative carbon materials capable of accommodating high metal dispersion. In order to achieve the highest Pt utilization efficiency, the Pt catalytic site must have simultaneous access to the fuel, the electron-conducting medium, and the proton-conducting medium. Carbon is a suitable material for supporting metallic nano particles in the electrode for DMFCs [4446]. Carbon material is the best choice owing to its essential properties such as electronic conductivity, corrosion resistance, surface properties, and the low cost required for the commercialization of fuel cells. The important requirements of carbon materials as supports for platinum-based electrocatalyst include (1) high surface area for a high degree of dispersion of nanosized catalysts, (2) good crystallinity or low electrical resistance to facilitate electron transport during the electrochemical reactions, (3) pore structure suitable for maximum fuel contact and byproduct release, and (4) good interactions between the catalyst nanoparticles and the carbon support. Recent research works have been focused to find out carbon materials with these properties.

So far the most widely used commercial carbon support is the Vulcan XC-72 (Cabot) with a BET surface area 240 m2 g1. However in order to achieve the highest catalyst utilization efficiency the carbon materials with a good electron conductivity and a well-controlled nanostructure involving high surface area together with a well developed porosity are developed and demonstrated as supports for fuel cells. 3.1.1. Carbon nanotubes (CNTs)

In recent years carbon nanotubes have been developed as supports for low-temperature fuel cells due to their unique structural, mechanical, and electrical properties. Carbon nanotubes have found to be promising materials to replace traditional carbon black powders [4751]. Xin research group has synthesized carbon nanotubes (CNTs) from high-purity graphite by a classical arc-discharge evaporation method (47). The BET surface area of the synthesized CNT is about 42 m2 g1 and Pt nanoparticles of spherical shape with an average particle sizes from 2 to 4 nm can be dispersed homogeneously. Using the Pt/CNT as cathode catalysts in DMFC single cell produces better performance compared to Vulcan XC-72 carbon. The high electro-catalytic activity may be attributed to the unique structure and better electronic properties of the CNTs, as well as to the specific interaction between Pt and CNTs. However, these results did not address the Pt utilization within the catalyst layer.

In a modified approach Yan et al have demonstrated the increased utilization of Pt by growing multiwalled carbon nanotube arrays directly on the carbon paper and then subsequently electrodepositing the Pt selectively on the carbon nanotubes [52]. They explained that this strategy can promise to improve the Pt utilization by securing the electronic route from Pt to the


supports in a PEMFC. The use of carbon nanotubes and the resulting feasible electronic pathway eliminate the problem of isolation of carbon particles from the electrode support due to the presence of Nafion used in conventional PEMFC strategies. This approach will improve the Pt utilization. The authors have reported that Pt catalyst loadings can be achievable up to 0.2 mg cm2 with the obtained MWNT-carbon.

Girishkumar et al have demonstrated a method to obtain films of varying amounts of single-walled carbon nano tubes (SWCNTs) on optically transparent electrode (OTE) surfaces using electrophoretic deposition. Platinum is electrodeposited on the SWCNT-OTE and the electrodes are studied for methanol oxidation as well as for oxygen reduction [53]. The high surface area and porosity of the SWCNT-OTE films enable them to use relatively small amounts of platinum and yet obtain excellent currents.

Multiwalled carbon nanotube-supported Pt (Pt/MWNT) nanocomposites were prepared by the aqueous solution reduction of a Pt salt (HCHO reduction), Pt/MWNT (A) and the reduction of a Pt ion salt in ethylene glycol solution, Pt/MWNT (B) by Liu et al [48]. The Pt/MWNT catalyst prepared by the EG method has a high and homogeneous dispersion of spherical Pt metal particles with a narrow particle-size distribution and the Pt particle size is in the range of 25 nm. From their results it is found that the surface chemical modifications of MWNTs and water content in EG solvent are the key factors in depositing Pt particles on MWNTs. In the case of the direct methanol fuel cell (DMFC) test, the Pt/MWNT catalyst prepared by EG reduction is slightly superior to the catalyst prepared by aqueous reduction and displays significantly higher performance than the Pt/XC-72 catalyst. The limited current density and maximum power density of the single cell with the synthesized Pt/MWNTs and Pt/XC-72 decrease in the following order: Pt/MWNT (B) > Pt/MWNT (A) > Pt/Carbon XC-72. The authors have concluded that by using MWNTs as supports the lower platinum loadings (1.0 mg/cm2) can be achieved. Also by lowering the agglomeration degree of metal particles in the Pt/MWNT (B) catalyst, the reactants easy access to the catalytic active sites, which would help to enhance the mass transport in the cell system further, can be improved. These differences in catalytic performance between the MWNT-supported or the carbon black XC-72-supported catalysts are attributed to a greater dispersion of the supported Pt particles when the EG method is used, in contrast to aqueous HCHO reduction and to possible unique structural and higher electrical properties when contrasting MWNTs to carbon black XC-72 as a support.

Liu et al have provided a method to deposit Pt nanoclusters of about 15 nm in diameter on MWCNTs by electroless deposition [49]. The MWCNTs are first pretreated with a solution of SnCl2/HCl and next placed in a platinum


electroless plating solution and MWCNTs with this plating showed higher electrocatalytic activity for oxygen reduction in a single stack PEM fuel cell.

Rajesh et al have demonstrated a method to place the catalyst on the inside of the CNTs. Their approach involves; first the CNTs are immersed in hexachloroplatinic acid and ruthenium chloride [54]. Then the incorporated metal ions are reduced to the corresponding metals by exposure in hydrogen gas at 823 K which yields a Pt or PtRu nanocluster loaded on CNT. In order to load the inside of the CNTs with WO3, which later will be bonded to Pt for lower activation energy in DMFCs; the CNT is immersed in peroxo tungstic acid. This immersion allows the peroxo-tungstic acid to penetrate inside the pores of the CNTs. Finally, the WO3/CNT structure is immersed in hexachloroplatinic acid, dried, and reduced with hydrogen gas at 623 K. Thus the result is a CNT with platinum tungsten-oxide nanoclusters inside.

The literature reports described in this section on CNTs as supports to noble metal particles have reached almost similar conclusions. Most of the researchers have concluded that the high electrocatalytic activities have been attributed to the: (i) high and homogeneous distribution of noble metal particles on CNTs, (ii) unique structural and high electrical properties, (iii) larger surface area provided by the CNT architecture, (iv) specific interaction between Pt and CNTs, and (iv) enhanced mass transport via facile access of reactants to the catalytically active site in the fuel cell operation. 3.1.2. Graphite carbon nanofibers (GCNFs)

Graphite nanofibers (GCNFs) have been explored as potential supports for Pt catalysts and metal supported GCNFs have been studied as electrodes for both oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) in single fuel cells [5557]. In their studies Bessel et al have examined three types of GCNFs as platinum support media during MOR. The GCNF materials possess, platelet, ribbon, and herring-bone structures [55].

These GNFs can be prepared by the catalytic decomposition of hydrocarbons over metal catalysts composed of copper, nickel, iron, or other bimetallic compounds. The advantage of this method is that metal nano particles can be supported on carbon with a preferred crystallographic orientation. For example when 5 wt% Pt is supported on platelet and ribbon type GNFs exposes mainly edge sites to the reactant during ORR. They have demonstrated that 5 wt% Pt on platelet and ribbon type GNFs exhibit improved oxidation activities of 400% when compared to the Vulcan XC-72 in methanol oxidation studies. They have explained the increased catalytic activity and lower self-poisoning ratios of the GNF supported Pt catalysts due to various reasons, including (a) more efficient mechanisms for the removal of the adsorbed species, (b) increased electrical conductance of the GNF when compared with the Vulcan carbon, (c) decreased impurities in carbon support,


and (d) the attainment of a preferred crystallographic orientation by the platinum particles as a result of the interaction with the highly ordered GNF substrate. Lukehart group has prepared a PtRu/herringbone GNF nanocomposite using a single-source molecular precursor as the metal source [56, 57]. The performance of the DMFC with this nanocomposite as the anode catalyst was enhanced by 50% relative to that recorded by an unsupported PtRu colloid anode catalyst. The enhanced performance of PtRu/herring bone GNF has been attributed to the specific support effects. 3.1.3. Carbon nanocoils (CNCs)

Carbon nanocoils (CNCs) composed of nanometer thick graphitic fibers have been successfully explored as electrode materials for DMFCs [5860]. Hyeon group has prepared the CNCs by simply heat-treating composites composed of a resorcinol-formaldehyde gel as a carbon precursor, silica, and a mixture of cobalt and nickel salts as a transition metal salt. The purpose of silica sol used in the preparation procedure is to obtain carbon materials with high surface area, and to achieve a good dispersion of the transition metal nanoparticles, which catalyze the formation of the graphitic nanostructure [58]. They have achieved high surface area of 318 m2 g1 with good crystallinity. PtRu alloy catalyst particles (60 wt%) with an average particle size of 2.3 nm are highly dispersed on CNCs. When applied as an anode material in DMFCs, these PtRu/CNCs have shown high electrocatalytic activity for methanol oxidation. The uniform distribution of the catalyst along with a small particle size, which is key factors for the stable and efficient operation of a DMFC seems to result from the good characteristics of CNCs, such as high crystallinity and large surface area. In addition, the unique pore structure of the support, which favors the diffusion of methanol fuel and the removal of by product CO2 gas, could be responsible for the improved fuel cell performance. 3.1.4. Porous carbon nanostructures

In search for novel nanocarbon materials as supports porous carbon tubule membranes are prepared by template-synthesis method [61]. The method involves the chemical-vapour deposition of carbon within the pores of alumina membranes. The carbon comprising these tubules can be transformed from a disordered material to highly ordered graphite. These C/alumina tubules can then be immersed into a solution of the desired metal ion and reduced to the corresponding metal or alloy by H2 gas. The underlying alumina can be removed by dissolving it in HF solution to obtain the desired free-standing carbon-tubule membrane supported electrode materials. Pt particles with an average particle size of about 7.1 nm and PtRu particles with particle size of about 1.7 nm can be dispersed on the synthesized carbon nanotubules. The


carbon nanotubule supported Pt electrodes have been studied for oxygen reduction.

Ryoo et al have showed that a periodic array of uniform ordered nanoporous carbon using the mesoporous aluminosilicate molecular sieves known as SBA-15 as templates [62]. These nanoporous carbon materials have tunable pore diameters and rigid structural order. The structure of the synthesized carbon is composed of ordered nanoporous carbon, which was originally formed inside the cylindrical nanotubes of the SBA-15 template. The ordered nanoporous carbon is rigidly interconnected into a highly ordered hexagonal array by carbon spacers even after SAB-15 removal. In this kind of nanostructured carbon, the pores or channels behave as individual nanoscale reactors so that chemical reactions are confined to take place inside the pores, with only limited diffusion between them.

They have found that even when the Pt loading is increased to the same weight of carbon (50 wt% Pt of the total weight), the Pt clusters show a very narrow particle-size distribution and average particle size of Pt cluster is around 2.5 nm. In the case of other porous carbons such as carbon black, activated charcoal and activated carbon fiber, the same experiments have resulted in the formation of much larger Pt particles with a wide distribution of diameters ranging up to 30 nm. Nanostructured carbon, capable of supporting Pt clusters with such high and uniform dispersion has been tested for O2 reduction. The mass activities obtained with the 2050 wt% Pt loadings are much higher than those of the Pt/carbon black samples onto which the Pt particles were supported, using the same procedure as for the ordered nanoprous carbon. The uniform distribution and decreased Pt cluster size using the nanostructured carbon as support is advantageous for DMFC applications.

Recently, Woo et al [63] have demonstrated a novel procedure to synthesize a new mesoporous platinum-carbon nanocomposite with a well control over particle size and shape. This method is based on the pyrolysis of carbon and platinum precursors in silica mesopores such as SBA-15 in order to take advantage of the excellent size- and shape-control achievable in the synthesis of nanodispersions. With the developed method they have synthesized Pt nanoclusters studded in the microporous nanowalls of ordered mesoporous carbon. They found that the materials is composed of regularly interconnected PtC nanocomposite arrays, and can be successfully used as a methanol tolerant cathode material in DMFC. A new type of periodically ordered bimodal porous carbon (POBPC) frame work with three-dimensionally interconnected, ordered, and uniform mesopores surrounded by mesostructured walls has been proposed as catalyst support in DMFCs [64]. The POBPC was determined to have a BET surface area of 465 m2 g1 and PtRu catalyst loadings to the tune of 80 wt% are possible. The methanol-oxidation activity of the POBPC supported PtRu


catalyst showed 79% high maximum power density than that of the E-TEK and Vulcan carbon-supported catalysts. The enhanced catalytic activity found with POBPC materials is attributed to their high surface areas and larger pore volumes of porous carbons which allow greater degree of catalyst dispersion. Due to the three-dimensionally interconnected macroscale and mesoscale bimodal porosity provides an open-highway network around the active catalyst for the facile diffusion of fuels and products to and from the catalyst. In the case of Vulcan carbon with its randomly distributed pores of varying size, respectively which may make fuel and product diffusion less efficient. Some of these carbon materials discussed above when applied as supports exhibit promising activities towards electrochemical reactions in DMFCs (Table 1).

Table 1. Summary of carbon supports for DMFCs under development.

3.2. Electrocatalysts 3.2.1. Electrocatalysts for methanol oxidation reaction

Electro-oxidation of methanol occurring at anode of DMFCs received great attention of both the academic and industrial researchers and a significant number of investigations have been carried out [65, 44]. To establish the best electrocatalyst composition, methanol oxidation on smooth electrode surfaces has been carried out in half-cell configuration. Electrochemical investigations coupled with spectroscopic techniques have been used to understand the oxidation mechanism and to find out the nature of the adsorbed species on the


electrode surfaces. Platinum is considered to be the best material for both the absorption and dehydrogenation of methanol [10]. The progressive dehydrogenation of the adsorbed methanol produce either linear or bridge-bonded carbon monoxide (CO) species which are strongly adsorbs on Pt surface (COads). COads species will poison the Pt electrode by blocking the active sites for methanol absorption and it is a significant impediment to the development of DMFCs [66]. Alloys with Pt have been sought as a means of enhancing the efficiency of methanol oxidation and this alloying route has achieved most successful results so far. PtRu was among the first binary systems to be studied and it remains the most effective [15, 67]. Alloying Pt with Sn also showed promising results for methanol oxidation and CO removal. 3.2.1.1. Binary Pt-based alloy electrocatalysts

The well-known anode deactivation due to adsorbed CO on Pt when methanol is used as fuel has encouraged to extensive research on platinum-based alloy catalysts, which have better CO tolerance than pure Pt. When Pt is alloyed with Ru, water dissociation occurs on Ru sites to produce RuOH groups at (at 0.2 0.3 V vs. RHE) less positive potentials than on pure Pt surface (0.7 V vs. RHE) [13] and the RuOH groups reacts with neighboring CO adsorbed on Pt to give carbon dioxide (See section 2.1. for detailed MOR mechanism on PtRu catalysts) according to the bifunctional mechanism [13, 14, 19, 20]. Another model proposed to explain the Ru promoting effect is the ligand model [2124]. The ligand model is based on the modification of Pt electronic structure by the presence of Ru rendering Pt atoms more susceptible for OH adsorption [68] or even dissociates adsorption of methanol [69]. Recently Waszczuk et al have shown that both bifunctional and ligand effect will be operative in PtRu catalysts [70]. For an efficient catalysis, it appears to be favorable to provide efficient adjacent sites for adsorption and desorption of reactant and product species. The presence of two complementary functioning catalytic sites in close proximity would seem to be a very favorable condition for the catalytic process of oxidation of methanol. Optimum Pt:Ru ratio: The influence of atomic bulk composition of PtRu catalysts towards methanol electro-oxidation was extensively studied and the subject is of much debate [7175]. The atomic composition of 50:50 (Pt:Ru) is found to be the optimum composition for methanol oxidation reaction at high temperature 90130 oC. According to Gasteiger et al, at room temperature, methanol is easily oxidized on PtRu catalysts with low (~10%) Ru content, but at intermediate temperature (for example at 60 oC) methanol oxidation occurs on PtRu catalysts with ~33% Ru content [73]. An increase in operation temperature would play a favorable role for the initial


dehydrogenation of methanol and it would facilitate the CO desorption, decreasing the coverage of irreversibly adsorbed species. However, Lamy and co-workers have found that, in a three electrode half cell as well as in a DMFC single cell, higher power densities were obtained with a PtRu atomic ratio of 80:20 irrespective of the temperature in the range from 20 to 110 oC. This controversy about the optimum composition of PtRu catalysts has not yet been resolved and needs further investigations. Catalyst preparation methods: PtRu/carbon catalyst, in which the PtRu alloy nanoclusters require several characteristics: (1) optimum compositions of at least 50 atomic percent Pt, (2) uniformity in alloy stoichiometry throughout the bulk sample, (3) well-defined surface structure, a preferred (111) face for structural effect, (4) an average particle size near 5 nm [76], and (5) total metal loadings sufficiently high to give acceptable performance in an operating DMFC, is a challenging goal. A variety of methods are available for the preparation of PtRu nanoparticles: The most common among them is the chemical reduction via sulfite-complex route [15, 77, 78]. Bnnemann and co-workers developed a new route using an organometallic compound to synthesize colloidal precursors for the PtRu/C catalyst, where the organic molecules are used to prevent to agglomeration and coalescence of the particles [79, 80]. High-area catalysts with completely alloyed bimetallic PtRu particles and a narrow particle size distribution with less than 3 nm diameter were prepared by adsorbing surfactant stabilized pre-formed PtRu colloids on high-surface area Vulcan XC 72 [81]. Impregnation method [56, 61, 8285], reverse micelles method [8688], microwave-assisted polyoly method and alcohol-reduction method [90] have also been available for the preparation of PtRu catalysts [89]. Recently we have produced a nano-sized PtRu/C catalysts via a modified alcohol-reduction method in which a small amount of Nafion is introduced during the preparation step [91]. The presence of Nafion is believed to enhance the activity of PtRu/C catalysts for the electro-oxidation of methanol by acting as a better dispersing agent and by increasing the ionic (proton) conductivity. With this method it is possible to prepare the PtRu/C catalysts with particle size of about 3 nm with a well dispersion over carbon (see figure 2).

Waszczuk et al have decorated platinum nanoparticles with ruthenium to obtain a Pt/Ru catalyst with a packing density of up to 0.65 Ru atoms per Pt surface atom by using spontaneous deposition method [92]. The activity of this catalyst toward methanol electrooxidation was tested at electrode potentials of interest for fuel cells. The catalyst activity maximizes at ruthenium packing density 0.4 0.5, and the catalyst activity was enhanced by two orders when compared to the commercial Pt/Ru alloy catalyst with 50:50 atomic compositions.


Figure 2. TEM image of PtRu/C catalyst prepared by a Nafion-stabilized alcohol-reduction method [91].

Recently, Nuzzo and Shapley have reported that the PtRu carbonyl

cluster complexes, PtRu5C(CO)16 and Pt2Ru4(CO)16 can serve as single-source molecular precursors for the preparation of PtRu5 and PtRu2/C nanocomposites [9395]. They prepared carbon-supported PtRu nanoparticles with a 1:5 Pt:Ru composition by the reductive condensation of a carbon-supported molecular cluster precursor PtRu5C(CO)16. During the H2 reduction process at 473 K the PtRu nanoparticles form a disordered structure in which Pt is found preferentially at the core of the condensing particle. After further high-temperature treatment to 673 K the nanoparticles adopt an inverted structure in which Pt appears preferentially at the surface of the bimetallic nanoparticle. With this process it is possible to prepare the PtRu nanoparticles with an average diameter of 1.5 nm. Lukehart et al have described the preparation of a Pt1-Ru1/carbon nanocomposite using (-C2H4)(Cl)Pt(-Cl)2-Ru(Cl)(3:3-2,7-dimethyloctadienediyl) as a noncluster, 1:1 Pt:Ru bimetallic molecular precursor [57]. Vulcan carbon (Cabot Corporation, XC72R) serves as the traditional carbon powder support. Microwave dielectric loss heating permits rapid conversion of precursor/carbon composites to the desired nanocomposites under appropriate conditions. This method can produce the PtRu nanocomposites with arbitrary total metal loadings of 16 and 50 wt% having metal alloy nanocrystals of 3.4 and 5.4 nm average diameters, respectively. The PtRu nanocomposites with 16 wt% metal loading shows catalytic performance comparable to that of related commercial catalysts


whereas the nanocomposite with 50 wt% shows higher performance when compared with the commercial supported catalysts having higher metal loadings. The summary of the preparation methods available for PtRu catalysts is shown in table 2. Role of Ru as a promoter metal: The promotional effect of Ru is explained by two effects: in bifunctional effect, the Ru sites acts as adsorption centres for the oxygen containing surface species formed by the dissociation of water (e.g. OH) at 0.20.3 V lower potentials than the pure Pt surface. The adsorbed oxygen-containing species on Ru are then reacts with CO to yield CO2. It is worthwhile to mention about the ensemble effect also. PtRu pair sites adsorb a more active form of oxygen-containing species rather than RuRu sites or Ru clusters. The optimum surface composition of Ru maximizes the PtRu pair sites within the constraints of the optimum ensemble for adsorption of the molecule. In the case of CO and HCOOH, adsorption is equally facile at the RuRu, PtPt and PtRu sites and the optimum surface composition is 50 at% Ru. For methanol, the situation is quite different as its adsorption occurs through consecutive dissociative steps. In terms of geometry, the optimum adsorption site seems to be a C3 Pt ensemble, and the composition which simultaneously maximizes the number of these ensembles and PtRu pairs is 10 at.% Ru [44, 9699] According to the ligand mechanism, Ru changes the

Table 2. Preparation methods for PtRu catalysts and their performance in DMFC.


electronic states of Pt, which affects both the activation of methanol CH bond and weakening of CO binding to Pt facilitates the CO2 removal. The increase in Pt d-band vacancies upon alloying with Ru as studied by X-ray absorption spectroscopy supports the ligand effect of Ru on Pt [100]. Increase in Pt d-band vacancies upon alloying with Ru can be evidenced in PtRu catalysts from the increase in white line area of Pt absorption edge from the X-ray absorption near-edge spectra (Figure 3).

Figure 3. Pt LIII-edge XANES spectra of a PtRu catalyst prepared by Nafion-stabilized alcohol-reduction method (solid line) and a Pt foil (line with triangles).

FTIR data on PtRu samples shows a CO stretching frequency shift to

higher wavenumbers with respect to Pt which is an evidence of Ru role in modifying the Pt electronic state [101]. In a similar study Frelink et al have observed a shift in linearly bonded CO stretching frequency to higher wave numbers at various coverages further supports the Ru effect on Pt electronic state [102, 14]. Differential electrochemical mass spectrometry (DEMS), a technique which provides unambiguous identification of transient species generated during electrochemical reaction, and in-situ ellipsometric studies on Pt and PtRu electrodes support the bifunctional mechanism [102]. From these in-situ experiments described above one can realize that the promotional effect of Ru could be due to the contribution of both the bifunctional and ligand effects. However, XPS results on PtRu dont show any change in Pt 4f spectra, indicating that the effect of Ru on Pt electronic state is less significant when compared to the bifunctional effect [103]. Wieckowski and co-workers have studied the relative magnitudes of the ligand and bifunctional effects by using 13C-NMR, temperature programmed desorption spectroscopy and cyclic voltammetry [104]. According to their results, Ru addition leads to a total


reduction in the overpotential by 170260 mV. Out of this total only 40 mV is due to the ligand effect and the remaining 130220 mV is contributed by the bifunctional effect. From all this work, one may conclude that the addition of Ru to Pt significantly increases the electrocatalytic activity of Pt through the adsorption of oxygenated species on Ru-sites. It becomes clear that PtRu catalysts are more effective for methanol oxidation since the reaction wants the electrocatalysts to be used in a potential regime where labile-bonded oxygen should be present on the surface. In this situation, the supply of active oxygen to the surface is of paramount importance since this, apparently, would facilitate the oxidation of adsorbed methanolic residues to CO2.

Alloying extent or atomic distribution in fuel cell catalysts: It has been fairly observed that our knowledge of the catalytic reaction mechanisms will be improved if we can relate the catalytic activity to the structural aspects of the materials because most of the catalytic reactions are structure sensitive and hence methods to get more insights into structural aspects are highly needed. Of interest is to control the homogeneity, dispersion, alloying extent and structure as they have profound influence on the surface properties which affect the catalytic activity, selectivity and stability of the bimetallic nanoparticles. Even though alloying is a well-known phenomenon, detailed studies on quantitative assessment of alloying extent (or) atomic distribution in bimetallic NPs have been lacking so far. Due to the constraints exists in choosing the number of atoms used in the simulation process it is hard to predict the structure and atomic distribution of NPs with so-called theoretical methods. Also the theoretical prediction that the bimetallic systems reach the thermodynamic equilibrium is not accessible during real-time synthesis of NPs. Even in case of bimetallic NPs having similar compositions, differences in atomic distribution, depending on the preparation conditions, will have strong influence on their catalytic properties. In our recent studies, we have explored a method which can estimate the atomic distribution of NPs which is most beneficial for physicochemical properties and their applications by X-ray absorption spectroscopy [105]. The extent of alloying of element A (JA) and element B (JB) for 1:1 AB bimetallic NPs can be calculated quantitatively by using the equations (1) and (2) respectively.

(1)

(2)


The parameters Pobserved, Robserved, Prandom, and Rrandom are defined as follows. The parameter Pobserved can be defined as a ratio of the scattering atoms B coordination number around absorbing A atoms (NAB) to the total coordination number of absorbing atoms (NAi), (Pobserved =NAB/NAi). Similarly, Robserved can be defined as a ratio of the scattering atoms A coordination number around absorbing B atoms (NBA) to the total coordination number of absorbing atoms (NBi), (Robserved = NBA/NBi). Whereas, Prandom and Rrandom can be taken as 0.5 for perfect alloyed bimetallic NPs if the atomic ratio of A and B is 1:1. This value can be achieved by assuming NAA = NAB and NBB = NBA which is generally true for perfect alloyed bimetallic NPs. In a similar way Prandom can be taken as 0.67 and 0.8 for 1:2 and 1:4 bimetallic NPs respectively. The parameter Rrandom can be taken as 0.33 and 0.2 for 1:2 and 1:4 bimetallic NPs respectively.

It is possible to construct the structural models emphasizing the atomic distribution in the bimetallic NPs with the knowledge of the NAi, NBi, JA and JB values derived from XAS. With the help of extent of alloying values and structural parameters extracted from EXAFS it is possible to predict the structure models of PtRu/C catalysts. We have calculated the alloying extent of Pt (JPt) and Ru (JRu) for commercial 30 wt% PtRu/C catalysts.

In case of JM 30 catalyst the coordination numbers of Pt and Ru atoms around the Pt atom are found to be 5.6 0.3 and 1.4 0.1, respectively, and the total coordination number NPti is 7.0. The coordination numbers of Ru and Pt atoms around the Ru atom are determined as 3.4 0.2 and 2.2 0.3, respectively, and the total coordination number NRui calculated as 5.6. From these values Pobserved and Robserved determined as 0.20 and 0.39, respectively, and JPt and JRu values are calculated as 40 and 78%, respectively. For E-TEK 30 catalyst we have calculated the coordination numbers of Pt and Ru atoms around the Pt atom as 6.2 0.3 and 0.9 0.1, respectively, and NPti as 7.1; the coordination numbers of Ru and Pt atoms around the Ru atom are determined as 3.7 0.2 and 1.2 0.2, respectively, and the NRui as 4.9. The other two structural parameters Pobserved and Robserved in the case of E-TEK 30 are calculated as 0.13 and 0.24, respectively, and the JPt and JRu values are calculated as 26 and 48%, respectively. It is clear from the structural coordination parameter values of both the catalysts that NPti > NRui and JRu > JPt, which indicates that the catalysts adopt a Pt rich in core and Ru rich in shell structure. A schematic representation of the catalyst structures is given in figure 4.

From the quantitative extent of alloying values, we can see that in both the catalysts considerable amount of Ru is segregated on the shell layer but the extent of segregation of Ru is higher in E-TEK 30 when compared to the JM 30. The increased value of JRu in JM 30 catalyst indicates that most of the Ru is involved in alloying and hence less segregation of Ru in the shell whereas in


Figure 4. Structural models deduced for JM 30 (a), and E-TEK 30 PtRu/C catalysts (b) based on XAS parameters. the case of ETEK 30 catalyst lesser extent of Ru is involved in the alloying and considerable extent of segregation of Ru can be expected in the shell region. The segregation of Ru in the case of ETEK 30 in part may be responsible for its lower methanol oxidation activity compared to JM 30. Recent infrared measurements on the PtRu alloy particle electrodes indicates two modes of adsorbed CO vibrations related to both Pt and Ru domains present on the surface supports the surface segregation of Ru in commercial catalysts [106]. The XAS results support the Pt-rich core and Ru rich shell structure for commercial carbon-supported PtRu catalysts. Increase in JPt and JRu values in JM 30 compared to E-TEK 30 indicates that the atomic distribution of Pt and Ru atoms are much facilitated. Increase in atomic distribution can be taken as a measure for enhanced homogeneity. 3.2.1.2. Binary Pt-based alloy electrocatalysts other than PtRu

PtSn system has found considerable interest, probably only second to that in PtRu system. Theoretical studies suggest that Sn alloyed into Pt is inactive in generating OHads [107]. In agreement with this, it was found experimentally that PtSn alloys are not active [108, 97], while electrosorbed or electrodeposited Sn on Pt is a reasonably good catalyst for methanol oxidation [109113]. However, the conclusion that PtSn alloys are inactive for methanol oxidation is disputed [114], even though the majority of experimental and theoretical evidence suggests otherwise. According to Wasmus and Kuver, this contradiction might well be an apparent one since (i) it is sometimes unclear whether a binary system is an alloy or just a mixture of two metals, and (ii) Sn may leach out under acid conditions being in turn readsorbed electrolytically at Pt sites [44]. Under less well controlled


experimental conditions, the latter effect may stimulate a catalytic activity of PtSn mixtures or alloys. Generally, it is thought that the adsorbed Sn on Pt is active while this is not the case for PtSn alloys might be due to the ionic nature of adsorbed Sn [111]. Neither chemisorbed nor electrosorbed foreign metals on Pt are a practical way for fuel cell catalysts since a load variation during fuel cell operation may lead to a change of the anode potential, resulting in desorption of the foreign metal so that its ions may diffuse into the electrolyte, and in turn to the cathode, which is a highly undesirable effect. PtRe [115], PtMo [116118], PtOs [119], PtWO3 [120, 121] and PtNi [122, 123] systems have been explored for methanol oxidation. For all these species, the determining factor for promotion is the formation of an adsorbed oxygen containing species on the secondary metal at potentials lower than for Pt. The oxygen containing species are needed for the oxidation of intermediate adsorbates. It is still necessary, however, to employ higher loadings for the catalysts than are needed for H2 oxidation. Other factors that influence the catalytic activity of the electrode are the support [124], the ionomer content in the active layer [125], and the fuel feed. It was found that the specific activity of supported PtRu/C is much higher than for a PtRu black. The maximum attainable voltage in cell is, however, much lower for the supported catalyst. The cell employing the unsupported catalyst also features a lower cross-over rate suggesting higher methanol utilization. The advantage of using a supported metal catalyst lies in the possibility to reduce the metal loadings drastically. The difference in performance may be due to the difference in morphology between the two types of catalyst. It is therefore necessary to improve the stability of both supported and unsupported metal catalysts [126, 127]. 3.2.1.3. Ternary Pt-based electrocatalysts

Mallouk et al have produced PtRuOs ternary alloys and predicted the improved methanol oxidation activity by considering the phase equilibria and relative PtC and MO (M = Ru, Os) bond strengths. From their studies it was found that Os is more oxophilic than Ru but significantly less soluble in face-centered cubic (fcc) Pt. The best catalytic activity was therefore found at ternary compositions near the Os solubility limit [128, 129]. They also have developed combinatorial screening method for ternary and quaternary alloy catalysts and found that the addition of small amounts of Ir to the PtRuOs ternary catalysts (Pt44Ru41Os10Ir5) significantly improved their performance [5]. The Os plays a role similar to the Ru where as the role of Ir appears to be accelerating CH bond activation processes. The other ternary systems developed for methanol oxidation are PtRuSn [130], PtRuW [131], PtRuSnW [132], PtRuSnW/C [133], PtRuMeOx (Me =W, Mo and V) [134], PtRuRh [135], and PtRuRhNi [136]. Some of these catalysts exhibit promising


properties towards methanol oxidation, however methods describing the role of third functionality in improving the catalytic activity are lacking so far. Even in the case of bimetallic systems, a detailed understanding of the metallic interactions and catalytic activity enhancement is also needed. For this a complete strategy is required in developing the catalyst systems and in this regard theoretical work [107, 137] coupled with combinatorial analysis [5] to improve screening efficiency are beneficial. To compare the catalytic activities of different catalysts, acceptable methods are needed to be developed. 3.2.2. Electrocatalysts for oxygen reduction reaction

In DMFCs the overpotential at the state-of-the-art Pt cathode is about 0.3 V even under open-circuit conditions. The high degree of irreversibility of the oxygen reduction reaction (ORR) is responsible for 0.2 V losses whereas short circuits in the cathode reaction resulted from the methanol cross-over causes 0.1 V losses. It means that about 25% potential losses from the theoretical maximum efficiency happen only in the cathode of a DMFC [138, 139]. The important strategies appeared in the literature to avoid the above mentioned problems are (i) the development of novel less-methanol permeable membranes, or modification of the existing membranes, (ii) use of methanol-tolerant ORR catalysts which are highly inactive towards methanol oxidation, and (iii) alloying Pt with transition metals such as Co, Ni, Fe, V, Mn and Cr.

Transition-metal macrocycles, namely transition-metal tetra methyl phenyl porphyrins (TMPPs) (such as FeTMPP, CoTMPP, and FeCoTMPP), and transition-metal tetra azaanulenes (such as CoTAA [140144], ruthenium based chalcogenides [145149] based on Chevrel-phase of the type (MoxRuySez) (0.02 < x < 0.04, 1 < y, z = 2y) [150], transition metal sulfides (MoxRuySz, MoxRhSz), or other transition metal chalcogenides (Ru1xMox) [151]) have been introduced as methanol tolerant oxygen cathodes because these compounds are inactive toward the oxidation of methanol. The ruthenium chalcogenides of type RuxXy (where x = S, Se, and Te) have also been explored as methanol-tolerant ORR catalysts [147]. Among these, the latter class of catalyst materials and, in particular, ruthenium-based cluster catalyst with selenium (RuSe) has been reported to be attractive for its selective catalytic activity towards ORR in the presence of methanol [152]. The tolerance of these materials to methanol is due to the absence of adsorption sites for methanol dehydrogenation. However, the intrinsic catalytic activities of these catalysts for the ORR are still lower than those of Pt-based catalysts, and the long-term stability under fuel cell operation at high potentials has not been well tested as compared to the Pt-based catalysts.

Alloying Pt with transition metals is a priming approach and this strategy not only improves the ORR activity but also decrease the cost. Many investigations have shown that Pt-based binary and ternary-alloy catalysts,


namely, PtCr/C [153155], PtFe/C [156, 157], PtNi/C [158], PtCo/C [158], and PtCoCr/C [159161] exhibits superior electrocatalytic activities towards ORR when compared to the Pt alone. Generally, most of the authors reported an activity enhancement of the ORR on the alloy catalysts by factors of 1.5 to 3 in comparison to pure Pt. The improvement in the ORR electrocatalysis on Pt-based alloy catalysts has been explained by several factors such as changes in short-range atomic order, particle size, Pt d-band vacancy, Pt skin effects and PtOH inhibition [162166]. More details about these electronic and structural effects are reviewed by Mukerjee [167] and Adzic [168]. Inhibiting the PtOH formation (approximately 0.8 V vs. RHE for Pt) will generally lower the overpotential losses by providing free sites for molecular oxygen adsorption. A number of prior reports have provided indirect evidence to the possibility of inhibiting the formation of anodic activation of water for PtOH formation [169171]. Shifting the onset potential for PtOH formation on Pt is dependent on (a) the ability of the alloying elements to modify the Pt electronic and short-range atomic order for inhibiting activation of H2O and (b) the ability of the alloying element to attract and hold H2Oads more strongly than the surrounding surface Pt atoms.

One of the difficulties in determining the effect of alloying components using supported catalysts is that the activity of a pure Pt supported catalysts can have a wide range of values depending on its microstructure and/or method of preparation. The intrinsic activity of Pt for the ORR depends on both particle shape and size [172, 173]. Since the alloyed Pt catalysts particles may not have either the same particle size or shape as the Pt catalysts to which they are compared, a simple comparison of activity normalized either by mass or surface area is insufficient to identify a true alloying effect and more detailed discussion of this point can be found in the literature [174]. The other important factors of the alloy clusters to obtain a good electrocatalytic activity are the dispersion and composition homogeneity [175, 176]. Significant improvement of the ORR catalysis on Pt-based alloy systems will require the inhibition of PtOH formation beyond 0.8 V.

Recently, Markovic group has studied the intrinsic catalytic activity of Pt3Ni and Pt3Co alloy catalysts with model bulk alloys characterized in UHV. The ORR of the Pt3Ni and Pt3Co catalysts has been studied in acid electrolytes using the rotating ring disk electrode (RRDE) method [177]. In their studies Pt3Ni and Pt3Co catalysts with two different surface compositions one with 75% Pt and the other with 100% Pt were prepared. They named the alloy with 100% Pt as Pt-skin structure and it was produced by an exchange of Pt and Co in the first two layers. They observed that the ORR activity order on these catalysts is dependent on the nature of the supporting electrolytes. It was found that the activity increases in the order Pt3Ni > Pt3Co > Pt in H2SO4. However, in HClO4 at 333 K, the ORR activity increases in the order Pt-skin > Pt3Co >


Pt3Ni > Pt. The catalytic enhancement of the ORR on Pt3Ni and Pt3Co vs. Pt is attributed to the inhibition of PtOHad formation on Pt sites surrounded by oxide-covered Ni and Co atoms beyond 0.8 V. Kinetic parameters for the ORR and the production of H2O2 on the Pt3Ni, Pt3Co, and Pt-skin alloys are the same as the pure Pt. The reaction order (m) was found to be one. The Tafel slope (90110 mV/dec) and the activation energy (20 to 25 kJ/mol) discerned for pure Pt is almost the same with values obtained for Pt3Ni and Pt3Co bimetallic surfaces. The fact that the same kinetic parameters are assessed from the analysis of the ORR data on all three surfaces implies that the reaction mechanism on Pt3Ni and Pt3Co alloy surfaces is the same as on proposed for pure Pt, i.e., a series 4 e reduction pathway.

Very recently, platinum monolayers supported on Au (111), Rh (111), Pd (111), Ru (001), and Ir (111) surfaces in HClO4 solutions have been studied as ORR catalysts [178]. The experimentally measured electrocatalytic activity of platinum monolayers for the ORR shows a volcano-type dependence on the centre of their d-bands as determined by density functional theory (DFT) calculations. The platinum monolayer supported on Pd(111) (PtML/Pd(111)) is at the top of the volcano curve and shows improved ORR activity over pure Pt (111). They have demonstrated this behavior by two opposite trends: while a higher lying d-band center tends to facilitate OO bond breaking, a lower lying one tends to facilitate bond formation (e.g., hydrogen addition).

In an another study Adzic group have demonstrated the kinetics of ORR in acid solutions on Pt monolayers deposited on a Pd (111) surface and on carbon-supported Pd nanoparticles using the rotating ring-disk electrode (RRDE) technique [179]. The kinetics of O2 reduction shows a significant enhancement at Pt monolayers on Pd (111) and Pd nanoparticle surfaces in comparison with the reaction on Pt (111) and Pt nanoparticles. They have suggested four-electron reduction mechanism for both the surfaces. The observed increase in the catalytic activity of Pt monolayer surfaces compared with Pt bulk and nanoparticle electrodes may reflect decreased formation of PtOH.

Development of Pt-alloys and Pt monolayers on Pd electrocatalysts are showing promising results towards oxygen reduction reaction. These catalysts enable a reduction of ORR overpotential losses by approximately 50 mV and a summary of these catalysts is shown in Table 3. The most important points to be considered for the development of ORR catalysts are:

(i) Particle size effects need to be studied carefully. The majority of

prior results on ORR activity variations with Pt cluster size showed a sharp drop in activity when cluster sized dropped below 2 nm. High-resolution transmission electron microscopy


Table 3. Summary of oxygen reduction reaction catalysts under development.

(HRTEM) analysis provides information on the particle size and shape.

(ii) Dispersion of metal crystallites in a conductive support like carbon is most important. The use of novel carbon materials can extend the electrode-electrolyte interface and thereby increase the catalyst and reactant utilization.

(iii) Preparation methodologies are needed to produce Pt alloys systems which can inhibit the formation of PtOH beyond 0.8 V.

(iv) Quantitative kinetic measurements for the ORR should be made to get more insights into the mechanism and activity. The rotating ring-disk electrode (RRDE) method allows the accurate determination of kinetic data such as Tafel slopes, reaction orders and (apparent) activation enthalpies in the absence of mass transport effects. Ring current measurements provide the parallel determination of the product distribution (H2O2) versus H2O formation under fuel cell relevant conditions and give insight to the reaction pathway.

4. Proton exchange membranes (PEMS)

DMFCs require membranes having reduced methanol permeability and water transport, which usually happen through diffusion and electro-osmotic drag. Solid polymer electrolytes such as perfluorosulfonic acid membranes (PFSA) (e.g., DuPonts Nafion) have been demonstrated [180183] in DMFCs. The PFSA membranes have a phase-separated structure comprising a hydrophobic matrix and interconnected hydrophilic cluster, called ionic


channels [184]. Proton conductance occurs through the ionic channels of the membranes [185]; these channels are formed by micro- or nanophase separation between the hydrophilic proton exchange sites and the hydrophobic domain. The Nafion membranes assure high proton conductivity ( 102 S.cm1) and high chemical stability. However during the DMFC operation the unreacted methanol at the anode can diffuse through the membrane and utilizes the cathode Pt sites for the direct reaction between methanol and oxygen, generating a mixed potential that reduces the cell voltage [186, 187]. Possible ways to mitigate this problem include using the thick Nafion 117 membranes (1100 EW, 7 mil ~ 178 m thick) and supplying the anode with dilute methanol, for example, 1 M or less to reduce the methanol crossover. However it is pointed out that the use of dilute methanol feed increases the systems complexity and reduces the energy density of the fuel, while the use of thick Nafion membrane increases the resistive losses of the cell [188]. Another way to overcome the methanol cross-over problem is to develop new polymer systems, or modify the existing membranes in order to achieve high ionic conductivity, low permeability to DMFC reactants, long-term stability under operating conditions and low cost [189].

There have been several attempts to explore non-perfluorinated polymers without the disadvantages of Nafion [190195]. Most of the works discussed about the incorporation of inorganic acid groups in to the basic polymer matrices like poly (sulfones)s, poly (ether ether ketone)s, poly (imides), polybenzimidazole, and polyacrylamide etc. The cost of these membranes is comparatively lower than the state-of-the-art Nafion membranes and properties are improved [196, 197]. Review articles dealing with the development of polymer-based proton conductors [198201], organicinorganic composites [202], polymer electrolyte membranes for fuel cells operating above 100 oC [203], and alternative polymer systems for proton exchange membranes (PEMs) [188] have been recently published.

The common requirements for proton exchange membranes include (1) high proton conductivity, (2) low electronic conductivity, (3) chemical stability, (4) thermal stability, (5) good mechanical properties in both the dry and hydrated states, (6) low water drag through diffusion and electro-osmosis, and (7) reasonable cost. In order to select the proper membrane for DMFC applications the most important parameters need to be characterized are proton conductivity, methanol permeability, and water swelling. The proton conductivity of the membranes can be evaluated by impedance spectroscopy [204, 205]. The methanol permeability can be estimated by pervaporation [206, 207] and diffusion cell experiments [208, 209]. Water swelling which gives a measure of the water solubility in the membranes is usually evaluated using batch experiments in liquid solutions at room temperature [210].


4.1. Sulfonated polyetheretherketones as an alternative to PFSA membranes

For developing non-Nafion membranes as proton exchange membranes (PEMs), the most widely investigated system is the sulfonation of polyetheretherketone (PEEK) [211214] or polyetheretherketoneketone (PEEKK) [215]. These polymers are believed to be having low cost and enhanced stability when compared to the perfluorinated polymer backbones [216]. The enhanced properties of sulfonated PEEK and PEEKK membranes when compared to the Nafion are qualitatively explained by differences in the microstructure and the acidity of the sulfonic acid functional groups [217].

In the case of perfluorosulfonic polymers the presence of water gives rise to some hydrophobic/hydrophilic nano-separation [217]. The sulfonic acid groups aggregate to form hydrophilic domain. In the presence of water only the hydrophilic domain of the nanostructure is hydrated to maintain the proton conductivity, while the hydrophobic domains provides the polymer with the morphological stability. However, in the case of SPEEK membranes the nano-separation of hydrophobic and hydrophilic domains is less pronounced. This is resulted from less hydrophobicity and smaller flexibility of the polymer backbone and the sulfonic acid group is less acidic and therefore less polar. As a result upon hydration the water molecules may be completely dispersed in the nanostructure of the sulfonated polymers [218]. The less hydrophobic nature of hydrocarbon backbone in SPEEK membranes may result in less dependence of conductivity on humidity in the low water activity range, allowing for good proton conductivity at high temperatures. Even at high water contents the water swelling in SPEEK membranes can be significantly improved by employing cross linking techniques such as ionic cross-linking by mixing with polymeric bases [219] and covalent cross-linking by applying sulfonate group containing polymers [220] or by blending with polybenzimidazole [221], heteropoly acids such as molybdophosphonic acid (H3PMo12O40xH2O) and tungstophosphoric acid (H3PW12O40xH2O) [222]. By incorporating heteropolyacids (HPA) into sulfonated PEEK polymer matrices, proton conductivity exceeded 102 S/cm at room temperature and reached values of 101 S/cm above 100 oC [193]. Increasing the degree of sulfonation also increases the proton conductivity of PEEK membranes, but higher sulfonation especially at higher temperatures leads to high swelling and poor mechanical properties of the membranes [223]. 4.2. Inorganic acid-doped polybenzimidazole (PBI) membranes

Inorganic mineral acids such as phosphoric acid, sulphuric acid can be doped into polybenzimidazole (PBI) polymeric electrolytes. These acid-doped


PBI membranes show excellent proton conductivities at temperatures up to 130150oC [224226]. Strong bases can also be doped into polybenzimidazole (PBI) and the resulting membrane show higher proton conductivity at temperatures above 100 oC [227, 228]. The proton conductivity of the acid-doped PBI membranes is dependent on the acid-doping level, temperature and humidity. For example He et al have observed proton conductivity about 2.5 102 S/cm at 200 oC for H3PO4-doped PBI membrane with 2 mol H3PO4 per repeat unit of PBI [229]. At increased H3PO4-doping level of about 5.7 mol per repeat unit of PBI, the proton conductivity is 4.6 102 S/cm at room temperature and increases to 7.9 102 S/cm at 200 oC. Due to the high proton conductivity at temperatures above 100 oC acid-doped PBI membranes are suitable for DMFC applications. 4.3. Polyphosphazene membranes

Owing to their better chemical and thermal stabilities polyphosphazene-based membranes have been explored for both hydrogen-air and DMFC applications [188]. Chemical attachment of various side chains for ion exchange sites and polymer cross-linking onto the PN polymer backbone is considered to be easy [230]. Pintauro et al. have shown that poly[(3-methylphenoxy)(phenoxy)phosphazene] and poly[bis(3-methyl phenoxy) phospazene] can be sulfonated by adding SO3 solution in dichloroethane solution dropwise to the polymer solution. A high polymer ion-exchange capacity of up to 2.0 mequiv/g was obtained when compared to the Dupont Nafion (IEC, 0.9 mmol/g) for the similar thicknesses of 0.2 mm [231, 232]. In another report, they described the fabrication of PEMs with sulfonate fixed charge sites from poly [bis(3-methyl phenoxy) phosphazene] [233]. To achieve sufficient mechanical properties polymer blending, cross-linking and other re-enforcement should be performed on these membranes. 4.4. Organicinorganic composite membranes

By incorporating inorganic proton conductors into the sulfonated hydrocarbon matrix, membranes with many interesting characteristics such as (i) low electro-osmotic drag, (ii) limited methanol cross-over, (iii) good mechanical strength, (iv) high proton conductivity, and (v) excellent thermal stability can be obtained. Polymer components without functional groups studied for organicinorganic composites include polyethylene oxides, PEO; polypropylene oxide, PPO; polytetramethylene oxide, PTMO [234238]; polybenzimidazole, PBI [239, 240]. Polymer components with functional groups such as sulfonated polysulfone, SPSF [241, 242]; sulfonated polystyrene, and sulfonated polyetheretherketone, SPEEK [243245].


Most of the organic-inorganic composite membranes can be prepared by the sol-gel process. Honma and co-workers have shown that nanosized silicate species can be prepared from organically modified alkoxysilane precursors. The nanosized silicate species are cross-linked with polyether polymers to get inorganic oxide in the polymer matrix. These organic-inorganic hybrid membranes become a proton conducting electrolyte by doping heteropolyacids such as 12-phosphotungstic acid (PWA) [234238]. By adopting the similar sol-gel approach, Lin and co-workers have synthesized organic-inorganic composite materials based on polyethyleneglycol (PEG)/SiO2 membranes for DMFC applications [246248]. The proton conductivity is achieved by doping acidic moieties of 4-dodecylbenzene sulfonic acid (DBSA) [248]. Other systems, such as phosphotungstic acid (PWA)-doped poly(vinyl alcohol) (PVA) [249, 250], PWA-doped PVA/SiO2 [251], polyacrylic acid (PAA)-doped PVA/SiO2 [252], and PWA-doped PEG/SiO2 [253], have also been prepared and tested as membranes for direct methanol fuel cells and promising proton conductivities have been achieved.

Polyvinylpyrrolidine (PVDF) polymers grafted with polystyrene (PSSA) by -radiation has been explored as DMFC membranes by Horsfall and co-workers [254]. They have reported the maximum power density of 22 mW/cm2 for DMFC single cell at 80 oC with the PVDF-PSSA membrane with 52% degree of grafting. In another report, PVDF-hexafluoropropylene (HFP) copolymer and Nafion blends have been studied for DMFCs applications [255]. Other organic-inorganic systems of interest reported so far are membranes containing tin-doped mordenities [256, 257], Zr-phosphonates [258], and zeolites [259]. Most of these membranes shows promising properties for DMFC applications, which are summarized in Table 4.

Very recently in our group we have focused on the development of silico-phosphate (SiO2P2O5) glass electrolytes as proton conductive materials [260, 261]. This kind of proton conductive electrolytes can be prepared by adopting an accelerated sol-gel process in which the precursors of tetraethyl-orthosilicate and trimethyl phosphate were partially hydrolyzed in the mixture of ethanol and water and then further hydrolyzed in the presence of formamide. The xerogels were then obtained by gelation at 60 oC for 3 days. With the controlled water/vapor management the gelation time needed is significantly shortened. The xerogels were sintered to get transparent glass electrolytes. In (SiO2P2O5) glass electrolyte protons are bonded with oxygen to form hydroxyl groups attaching to network-forming cations such as Si4+ and P5+. Figure 5 shows the network structure of the synthesized glass electrolytes and the bonding environments of the water molecule and hydroxyl group on the network structure derived from the in situ FTIR observations.

The SiO2P2O5 glass electrolyte proton conductivity is found to be insensitive to humidity and it is an advantage when compared to the


Table 4. Properties of some proton exchange membranes under development.

Figure 5. Proposed network structure of (SiO2P2O5) glass electrolytes [260].


characteristics of Nafion. The reason may be ascribed to that the mechanisms of proton transport in the SiO2P2O5 glass electrolyte and in Nafion are different. In the case of Nafion the water-filled pores in the membrane form a network of pathways for proton transport. Protons jumps between hydrated sulfonic acid groups fixed on a stable polymer host and aqueous domains. Membranes with high degrees of hydration exhibit substantial proton mobility rationalized on the basis of structural diffusion mechanism resembling that in bulk-water. At low water content, proton transports via the segmental motions of polymer. In the matrix of Nafion, there are clusters of water molecules and ions separated by polymer, and the proton conductivity will take place when ions move from cluster to cluster via the polymeric segments in the region between the clusters and the mechanism was named as a hopping mechanism [249]. In the case of SiO2P2O5 glass electrolyte membranes, the conductivity changes insignificantly with relative humidity at the temperature range of 7080 oC, which suggests that the proton conducting mechanism in the glass electrolyte membranes is different from that of Nafion. The conduction of protons in the glass electrolytes is mainly through the hydrogen bonding water and hydroxyl group on the network structure, like hopping. The protons move from one bonding water or hydroxyl group to another one. We have achieved the proton conductivity of about 9.45 103 S/cm at 70 oC and 100% relative humidity. Methanol permeability of SiO2P2O5 glass electrolyte membranes was found to be 2.1 109 cm2/s and this value is about three orders less than that of the state-of-the-art Nafion membrane (1.57 106 cm2/s) indicating that methanol cross-over can be reduced if we employ SiO2P2O5 glass membranes in DMFCs.

The membrane materials which are suitable to operate at high temperatures with high proton conductivity are the newest developments in the membrane research. The recent progress in this field reveal that membranes based on modification of state-of-the-art Nafion, non-Nafion systems like sulfonated polyetheretherketones (SPEEKs), inorganic acid-doped polybenzimidazole (PBI), polyphosphazenes, organic-inorganic composite, and SiO2P2O5 glass electrolytes exhibit promising conductivities and most systems are successfully operated at temperatures above 100 oC for DMFC applications.

Figure 6 shows the proton conductivity and methanol permeability of some of the membranes reported in the literature. The methanol permeabilities through PEM were proportional to the proton conductivities. The ideal membrane should possess high proton conductivity with low methanol permeability and thus the location of the membrane should be in the upper left-hand corner. In the case of membranes in which the proton transport mechanism is dominated by the surface hopping then the proton conductivity of the membrane will be higher with much less degree of methanol


permeability. However in the case of membranes in which the proton transport is dominated by bulk diffusion then the methanol permeability will be significantly increased.

Figure 6. Log proton conductivity versus log methanol permeability of PEMs developed for DMFC applications. 5. Conclusions

There is still a clear need for the synthesis and complete characterization of new carbon supports, electrocatalysts and proton exchange membrane materials for DMFCs. Herein we have reviewed the recent progress achieved in the development of materials for DMFCs. In the search for carbon materials, focus has been made in attaining well-ordered porosity, high surface area, and good mechanical strengths. In recent years, mesostructured carbon materials are emerging as promising supports for fuel cell catalysts because the pore structure of the carbons can be tailored as a function of the silica that is used as template. Other important factors to be considered in the development of carbon materials are dispersion effect, and electronic conductivity. Regarding the electrocatalysts stress should be made on the evaluation of better catalysts other than PtRu catalysts, need to achieve higher loadings on carbon supports, and care should be taken in comparing the catalytic activities of different electrocatalysts. It is difficulty to compare the activity of different electrocatalysts appeared in the literature as there is no standard evaluation method available. Active surface area normalization procedures are beneficial in this regard. Methodologies are required to determine the extent of alloying and atomic distribution in fuel cell catalysts as these properties have profound influence on the surface properties which affect the catalytic activity,


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advances in fuel cells - chapter 2

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methanol fuel availability

development of dmfcs

optimum fuel

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low methanol permeability

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